UNIT 1
DIVERSITY IN THE LIVING WORLD
Chapter 1
The Living World
Chapter 2
Biological Classification
Chapter 3
Plant Kingdom
Chapter 4
Animal Kingdom
Biology is the science of life forms and living processes. The living world
comprises an amazing diversity of living organisms. Early man could
easily perceive the difference between inanimate matter and living
organisms. Early man deified some of the inanimate matter (wind, sea,
fire etc.) and some among the animals and plants. A common feature of
all such forms of inanimate and animate objects was the sense of awe
or fear that they evoked. The description of living organisms including
human beings began much later in human history. Societies which
indulged in anthropocentric view of biology could register limited
progress in biological knowledge. Systematic and monumental
description of life forms brought in, out of necessity, detailed systems
of identification, nomenclature and classification. The biggest spin off
of such studies was the recognition of the sharing of similarities among
living organisms both horizontally and vertically. That all present day
living organisms are related to each other and also to all organisms
that ever lived on this earth, was a revelation which humbled man and
led to cultural movements for conservation of biodiversity. In the
following chapters of this unit, you will get a description, including
classification, of animals and plants from a taxonomist’s perspective.
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Born on 5 July 1904, in Kempten, Germany, ERNST MAYR, the
Harvard University evolutionary biologist who has been called
‘The Darwin of the 20th century’, was one of the 100 greatest
scientists of all time. Mayr joined Harvard’s Faculty of Arts
and Sciences in 1953 and retired in 1975, assuming the title
Alexander Agassiz Professor of Zoology Emeritus. Throughout
his nearly 80-year career, his research spanned ornithology,
taxonomy, zoogeography, evolution, systematics, and the
history and philosophy of biology. He almost single-handedly
made the origin of species diversity the central question of
evolutionary biology that it is today. He also pioneered the
currently accepted definition of a biological species. Mayr was
awarded the three prizes widely regarded as the triple crown of
biology: the Balzan Prize in 1983, the International Prize for
Biology in 1994, and the Crafoord Prize in 1999. Mayr died at
the age of 100 in the year 2004.
Ernst Mayr
(1904 – 2004)
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C HAPTER 1
THE LIVING W ORLD
1.1 Diversity in the
Living World
1.2 Taxonomic
Categories
How wonderful is the living world ! The wide range of living types is
amazing. The extraordinary habitats in which we find living organisms,
be it cold mountains, deciduous forests, oceans, fresh water lakes, deserts
or hot springs, leave us speechless. The beauty of a galloping horse, of
the migrating birds, the valley of flowers or the attacking shark evokes
awe and a deep sense of wonder. The ecological conflict and cooperation
among members of a population and among populations of a community
or even the molecular traffic inside a cell make us deeply reflect on – what
indeed is life? This question has two implicit questions within it. The first
is a technical one and seeks answer to what living is as opposed to the
non-living, and the second is a philosophical one, and seeks answer to
what the purpose of life is. As scientists, we shall not attempt answering
the second question. We will try to reflect on – what is living?
1.1 D IVERSITY IN THE LIVING WORLD
If you look around you will see a large variety of living organisms, be it
potted plants, insects, birds, your pets or other animals and plants. There
are also several organisms that you cannot see with your naked eye but
they are all around you. If you were to increase the area that you make
observations in, the range and variety of organisms that you see would
increase. Obviously, if you were to visit a dense forest, you would probably
see a much greater number and kinds of living organisms in it. Each
different kind of plant, animal or organism that you see, represents a
species. The number of species that are known and described range
between 1.7-1.8 million. This refers to biodiversity or the number and
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types of organisms present on earth. We should remember here that as
we explore new areas, and even old ones, new organisms are continuously
being identified.
As stated earlier, there are millions of plants and animals in the world;
we know the plants and animals in our own area by their local names.
These local names would vary from place to place, even within a country.
Probably you would recognise the confusion that would be created if we
did not find ways and means to talk to each other, to refer to organisms
we are talking about.
Hence, there is a need to standardise the naming of living organisms
such that a particular organism is known by the same name all over the
world. This process is called nomenclature. Obviously, nomenclature
or naming is only possible when the organism is described correctly and
we know to what organism the name is attached to. This is identification.
In order to facilitate the study, number of scientists have established
procedures to assign a scientific name to each known organism. This is
acceptable to biologists all over the world. For plants, scientific names are
based on agreed principles and criteria, which are provided in International
Code for Botanical Nomenclature (ICBN). You may ask, how are animals
named? Animal taxonomists have evolved International Code of Zoological
Nomenclature (ICZN). The scientific names ensure that each organism
has only one name. Description of any organism should enable the people
(in any part of the world) to arrive at the same name. They also ensure
that such a name has not been used for any other known organism.
Biologists follow universally accepted principles to provide scientific
names to known organisms. Each name has two components – the
Generic name and the specific epithet. This system of providing a
name with two components is called Binomial nomenclature. This
naming system given by Carolus Linnaeus is being practised by biologists
all over the world. This naming system using a two word format was
found convenient. Let us take the example of mango to understand the
way of providing scientific names better. The scientific name of mango is
written as Mangifera indica. Let us see how it is a binomial name. In this
name Mangifera represents the genus while indica, is a particular species,
or a specific epithet. Other universal rules of nomenclature are as follows:
1.
Biological names are generally in Latin and written in italics.
They are Latinised or derived from Latin irrespective of their
origin.
2.
The first word in a biological name represents the genus while
the second component denotes the specific epithet.
3.
Both the words in a biological name, when handwritten, are
separately underlined, or printed in italics to indicate their Latin
origin.
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THE LIVING WORLD
4.
5
The first word denoting the genus starts with a capital letter
while the specific epithet starts with a small letter. It can be
illustrated with the example of Mangifera indica.
Name of the author appears after the specific epithet, i.e., at the end of
the biological name and is written in an abbreviated form, e.g., Mangifera
indica Linn. It indicates that this species was first described by Linnaeus.
Since it is nearly impossible to study all the living organisms, it is
necessary to devise some means to make this possible. This process is
classification. Classification is the process by which anything is grouped
into convenient categories based on some easily observable characters.
For example, we easily recognise groups such as plants or animals or
dogs, cats or insects. The moment we use any of these terms, we associate
certain characters with the organism in that group. What image do you
see when you think of a dog ? Obviously, each one of us will see ‘dogs’
and not ‘cats’. Now, if we were to think of ‘Alsatians’ we know what we are
talking about. Similarly, suppose we were to say ‘mammals’, you would,
of course, think of animals with external ears and body hair. Likewise, in
plants, if we try to talk of ‘Wheat’, the picture in each of our minds will be
of wheat plants, not of rice or any other plant. Hence, all these - ‘Dogs’,
‘Cats’, ‘Mammals’, ‘Wheat’, ‘Rice’, ‘Plants’, ‘Animals’, etc., are convenient
categories we use to study organisms. The scientific term for these
categories is taxa. Here you must recognise that taxa can indicate
categories at very different levels. ‘Plants’ – also form a taxa. ‘Wheat’ is
also a taxa. Similarly, ‘animals’, ‘mammals’, ‘dogs’ are all taxa – but you
know that a dog is a mammal and mammals are animals. Therefore,
‘animals’, ‘mammals’ and ‘dogs’ represent taxa at different levels.
Hence, based on characteristics, all living organisms can be classified
into different taxa. This process of classification is taxonomy. External
and internal structure, along with the structure of cell, development
process and ecological information of organisms are essential and form
the basis of modern taxonomic studies.
Hence, characterisation, identification, classification and nomenclature
are the processes that are basic to taxonomy.
Taxonomy is not something new. Human beings have always been
interested in knowing more and more about the various kinds of
organisms, particularly with reference to their own use. In early days,
human beings needed to find sources for their basic needs of food, clothing
and shelter. Hence, the earliest classifications were based on the ‘uses’ of
various organisms.
Human beings were, since long, not only interested in knowing more
about different kinds of organisms and their diversities, but also the
relationships among them. This branch of study was referred to as
systematics. The word systematics is derived from the Latin word
‘systema’ which means systematic arrangement of organisms. Linnaeus
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used Systema Naturae as the title of his publication. The scope of
systematics was later enlarged to include identification, nomenclature
and classification. Systematics takes into account evolutionary
relationships between organisms.
1.2
TAXONOMIC CATEGORIES
Classification is not a single step process but involves hierarchy of steps
in which each step represents a rank or category. Since the category is a
part of overall taxonomic arrangement, it is called the taxonomic category
and all categories together constitute the taxonomic hierarchy. Each
category, referred to as a unit of classification, in fact, represents a rank
and is commonly termed as taxon (pl.: taxa).
Taxonomic categories and hierarchy can be illustrated by an example.
Insects represent a group of organisms sharing common features like
three pairs of jointed legs. It means insects are recognisable concrete
objects which can be classified, and thus were given a rank or category.
Can you name other such groups of organisms? Remember, groups
represent category. Category further denotes rank. Each rank or taxon,
in fact, represents a unit of classification. These taxonomic groups/
categories are distinct biological entities and not merely morphological
aggregates.
Taxonomical studies of all known organisms have led to the
development of common categories such as kingdom, phylum or division
(for plants), class, order, family, genus and species. All organisms,
including those in the plant and animal kingdoms have species as the
lowest category. Now the question you may ask is, how to place an
organism in various categories? The basic requirement is the knowledge
of characters of an individual or group of organisms. This helps in
identifying similarities and dissimilarities among the individuals of the
same kind of organisms as well as of other kinds of organisms.
1.2.1
Species
Taxonomic studies consider a group of individual organisms with
fundamental similarities as a species. One should be able to distinguish
one species from the other closely related species based on the distinct
morphological differences. Let us consider Mangifera indica, Solanum
tuberosum (potato) and Panthera leo (lion). All the three names, indica,
tuberosum and leo, represent the specific epithets, while the first words
Mangifera, Solanum and Panthera are genera and represents another
higher level of taxon or category. Each genus may have one or more than
one specific epithets representing different organisms, but having
morphological similarities. For example, Panthera has another specific
epithet called tigris and Solanum includes species like nigrum and
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THE LIVING WORLD
7
melongena. Human beings belong to the species sapiens which is grouped
in the genus Homo. The scientific name thus, for human being, is written
as Homo sapiens.
1.2.2
Genus
Genus comprises a group of related species which has more characters
in common in comparison to species of other genera. We can say that
genera are aggregates of closely related species. For example, potato and
brinjal are two different species but both belong to the genus Solanum.
Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with several
common features, are all species of the genus Panthera. This genus differs
from another genus Felis which includes cats.
1.2.3
Family
The next category, Family, has a group of related genera with still less
number of similarities as compared to genus and species. Families are
characterised on the basis of both vegetative and reproductive features of
plant species. Among plants for example, three different genera Solanum,
Petunia and Datura are placed in the family Solanaceae. Among animals
for example, genus Panthera, comprising lion, tiger, leopard is put along
with genus, Felis (cats) in the family Felidae. Similarly, if you observe the
features of a cat and a dog, you will find some similarities and some
differences as well. They are separated into two different families – Felidae
and Canidae, respectively.
1.2.4
Order
You have seen earlier that categories like species, genus and families are
based on a number of similar characters. Generally, order and other
higher taxonomic categories are identified based on the aggregates of
characters. Order being a higher category, is the assemblage of families
which exhibit a few similar characters. The similar characters are less in
number as compared to different genera included in a family. Plant
families like Convolvulaceae, Solanaceae are included in the order
Polymoniales mainly based on the floral characters. The animal order,
Carnivora, includes families like Felidae and Canidae.
1.2.5
Class
This category includes related orders. For example, order Primata
comprising monkey, gorilla and gibbon is placed in class Mammalia along
with order Carnivora that includes animals like tiger, cat and dog. Class
Mammalia has other orders also.
1.2.6
Phylum
Classes comprising animals like fishes, amphibians, reptiles, birds along
with mammals constitute the next higher category called Phylum. All
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these, based on the common features like presence of notochord
and dorsal hollow neural system, are included in phylum
Chordata. In case of plants, classes with a few similar characters
are assigned to a higher category called Division.
1.2.7 Kingdom
Figure 1.1 T a x o n o m i c
categories
s h o w i n g
hierarchial
arrangement
in ascending
order
All animals belonging to various phyla are assigned to the
highest category called Kingdom Animalia in the classification
system of animals. The Kingdom Plantae, on the other hand, is
distinct, and comprises all plants from various divisions.
Henceforth, we will refer to these two groups as animal and
plant kingdoms.
The taxonomic categories from species to kingdom have been
shown in ascending order starting with species in Figure 1.1.
These are broad categories. However, taxonomists have also
developed sub-categories in this hierarchy to facilitate more
sound and scientific placement of various taxa.
Look at the hierarchy in Figure 1.1. Can you recall the basis
of arrangement? Say, for example, as we go higher from species
to kingdom, the number of common characteristics goes on
decreasing. Lower the taxa, more are the characteristics that the
members within the taxon share. Higher the category, greater is
the difficulty of determining the relationship to other taxa at the
same level. Hence, the problem of classification becomes more
complex.
Table 1.1 indicates the taxonomic categories to which some
common organisms like housefly, man, mango and wheat belong.
TABLE 1.1 Organisms with their Taxonomic Categories
Common
Name
Biological
Name
Man
Genus
Family
Order
Class
Phylum/
Division
Homo sapiens Homo
Hominidae
Primata
Mammalia
Chordata
Housefly
Musca
domestica
Musca
Muscidae
Diptera
Insecta
Arthropoda
Mango
Mangifera
indica
Mangifera Anacardiaceae Sapindales Dicotyledonae
Wheat
Triticum
aestivum
Triticum
Poaceae
Poales
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Angiospermae
Monocotyledonae Angiospermae
THE LIVING WORLD
9
S UMMARY
The living world is rich in variety. Millions of plants and animals have been
identified and described but a large number still remains unknown. The
very range of organisms in terms of size, colour, habitat, physiological and
morphological features make us seek the defining characteristics of living
organisms. In order to facilitate the study of kinds and diversity of organisms,
biologists have evolved certain rules and principles for identification,
nomenclature and classification of organisms. The branch of knowledge
dealing with these aspects is referred to as taxonomy. The taxonomic studies
of various species of plants and animals are useful in agriculture, forestry,
industry and in general for knowing our bio-resources and their diversity.
The basics of taxonomy like identification, naming and classification of
organisms are universally evolved under international codes. Based on the
resemblances and distinct differences, each organism is identified and
assigned a correct scientific/biological name comprising two words as per
the binomial system of nomenclature. An organism represents/occupies a
place or position in the system of classification. There are many categories/
ranks and are generally referred to as taxonomic categories or taxa. All the
categories constitute a taxonomic hierarchy.
E XERCISES
1.
2.
3.
Why are living organisms classified?
Why are the classification systems changing every now and then?
What different criteria would you choose to classify people that you
meet often?
4. What do we learn from identification of individuals and populations?
5. Given below is the scientific name of Mango. Identify the correctly
written name.
Mangifera Indica
Mangifera indica
6. Define a taxon. Give some examples of taxa at different hierarchical
levels.
7. Can you identify the correct sequence of taxonomical categories?
(a)
Species
Order
Phylum
Kingdom
(b)
Genus
Species
Order
Kingdom
(c)
Species
Genus
Order
Phylum
8. Try to collect all the currently accepted meanings for the word ‘species’.
Discuss with your teacher the meaning of species in case of higher
plants and animals on one hand, and bacteria on the other hand.
9. Define and understand the following terms:
(i) Phylum (ii) Class (iii) Family (iv) Order (v) Genus
10. Illustrate the taxonomical hierarchy with suitable examples of a plant
and an animal.
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